1. Field of the Invention
This invention relates to the field of biological specimen mounting, and more particularly to adhesives for such use.
2. Description of the Prior Art
Very thin slices of animal and plant tissue are prepared for many different kinds of microscopic studies by sectioning with a variety of kinds of microtomes. The tissue may be cut fresh. The soft and compliant nature of most fresh tissue makes it very difficult to cut undistorted thin sections. If the tissue is frozen and is cut on a freezing microtome or in a cryostat temperatures below 0.degree. C. (32.degree. F.), the hardness of the frozen water within the tissue permits sections to be cut relatively easily as thin as a few micrometers. However, these sections are brittle and friable and therefore difficult to handle and process further. To make sectioning of tissue still easier, a number of procedures produce a-block of supported tissue which has superior sectioning properties and produce high quality, relatively easy-to-handle tissue sections. Such procedures typically involve: (1) Fixation of the tissue in a solution which insolubilizes the natural polymers of which tissue cells are composed, and which hardens them; (2) dehydration through a series of water-miscible (e.g., an alcohol) and then paraffin or plastic-monomer-miscible (e.g., toluene or xylene) solvents; (3) infiltration in melted paraffin or monomer solution; and (4) embedding by freezing the paraffin or polymerizing the monomer to form a solid polymer. [Staining Methods, J. F. S. McManus and R. W. Mowry (P. B. Hoeber, Inc., N.Y. 1960); Techniques for Electron Microscopy, D. Kay, Ed. (Blackwell Sci. Publ., Oxford, England, 1965) pp. 166-212.]
However, there are occasional specimens which remain difficult to section. As the section is cut, the parts of the cut section often tend to fragment and fall from the cut section, or fall from the section as it is removed from the microtome.
A. Palmgren, Nature, Vol. 174, p. 46 (1954), introduced the use of a conventional adhesive tape as a sectioning aid for the cutting of very large, hard or brittle specimens. A piece of adhesive tape is applied to the surface of a cut frozen or paraffin block, just before the specimen is advanced the incremental thickness of a (next) section. The microtome advances and the next section is cut and is thus supported by the applied tape. The quality of the uncompressed section of hard brittle and friable tissue produced by this means can be far superior to that of a conventional section of the same block of tissue. However, following Palmgren, processing such a section while it remains on the tape, or transferring it to a glass slide (to permit it to be processed thereafter in a conventional way) involved elaborate, time-consuming and inconvenient methods, which are also potentially damaging to the section. Palmgren's method, therefore, did not become popular.
W. E. Beckel, Nature, Vol. 184, p. 1584 (1959) introduced the use of Scotch brand No. 810 cellulose-acetate-backed adhesive tape in Palmgren's process. The tape-mounted sections were applied, section-side down on conventionally wet albuminized glass slides. After thorough drying for a few hours, the adhesive backing, the adhesive layer and the paraffin were all dissolved in tetrahydrofuran in 30 minutes, leaving the section adhering to the glass slide, and available for further processing by conventional techniques. Alternatively, chloroform for 2 minutes, followed by xylene for 30 minutes, worked equally well. Beckel also described a "more rapid method" which used a film of albumin and a solution of 2 per cent celloidin in methyl benzoate or ethyl alcohol to "cement" the section to the glass slide followed by 1 minute in chloroform and 10 minutes in xylene to complete the treatment. Some other equally time-consuming variations were described.
D. S. Gowers and R. E. Miller, Nature, Vol. 190, p. 425 (1961), attempted to repeat Beckel's method but found that, with available Scotch brand No. 810 tape, the adhesive could not be dissolved, and with the best alternate available tape, Tuck brand No. 200, safe removal of the tape without damaging the section in solvent took from 1 to 10 hours.
R. P. Wedeen and H. I. Jernow, Am. J. Physiol., Vol. 214, p. 776 (1968) used cyanoacrylate (Eastman 910 "super-glue") to attach adhesive-tape-supported frozen sections to radioautographic (photographic) plates. The cyanoacrylate is initially liquid, but polymerizes to a solid when it is squeezed out into a thin film. The cyanoacrylate polymer is soluble in xylene and other processing solvents, which would cause the section to float free, and so is not useful for ordinary use.
In another area of study relating to the field of specimen section handling, various resinous mounting media have been developed. After the tissue section has been stained, the slide is transferred from water through alcoholic and then resincompatible solvents, such as xylene, and the section is mounted in a resinous medium chosen to have a refractive index which closely approximates the average refractive index of the unstained tissue specimens, e.g., a refractive index from about 1.530 to about 1,570. Such are described, for example, in U.S. Pat. No. 4,120,991, and McManus & Mowry, above.